Robust OOP & Error Handling

This is Tier 4, where the object model becomes complete and Java turns toward modern, functional, and well-structured code. You'll specialize types with inheritance and watch polymorphism pick the right behavior at run time; define contracts with abstract classes and interfaces; handle failure deliberately with exceptions; turn behavior into values with lambdas; model closed sets of data with sealed types and consume them with pattern matching; and learn how packages, modules, and build tools organize and ship real code.

Six chapters, in order:

1. Inheritance & Polymorphism — extends, overriding, super, dynamic dispatch, final, and the Object methods.
2. Abstract Classes & Interfaces — abstract methods, interfaces, default methods, and one-class/many-interfaces.
3. Exceptions — checked vs unchecked, try/catch/finally, custom exceptions, and try-with-resources.
4. Nested & Anonymous Classes; Lambdas — nested classes, anonymous classes, lambdas, functional interfaces, and method references.
5. Sealed Classes & Pattern Matching — sealed, pattern matching for instanceof, switch patterns, and record patterns.
6. Packages, Modules & the Build — packages and the classpath, JPMS modules, JARs, and Maven/Gradle.

Every code block with a ▶ Run button is live; the project-level examples in the final chapter are shown as real terminal sessions. The highest-value habit at this tier is to ask "what's the right tool for this shape of problem" — inheritance vs interfaces vs sealed types each fit a different situation, and choosing well is most of good design.